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<header id="nav"><a
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 class="beginner" href="A6-MemoryMap.html">Memory Map</a><a
 class="advanced" href="B1-RemoteTargets.html">Remote Targets</a><a
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</header>
<header id="title-block-header">
<h1 class="title">Ghidra Debugger</h1>
</header>
<nav id="TOC" role="doc-toc">
<ul>
<li><a href="#emulation" id="toc-emulation">Emulation</a>
<ul>
<li><a href="#p-code-emulation-and-caveats"
id="toc-p-code-emulation-and-caveats">P-code Emulation and
Caveats</a></li>
<li><a href="#use-cases" id="toc-use-cases">Use Cases</a></li>
<li><a href="#extrapolation-and-interpolation"
id="toc-extrapolation-and-interpolation">Extrapolation and
Interpolation</a>
<ul>
<li><a href="#stepping-schedules" id="toc-stepping-schedules">Stepping
Schedules</a></li>
<li><a href="#exercise-demonstrate-the-cell-numbers"
id="toc-exercise-demonstrate-the-cell-numbers">Exercise: Demonstrate the
Cell Numbers</a></li>
</ul></li>
<li><a href="#emulating-a-program-image"
id="toc-emulating-a-program-image">Emulating a Program Image</a>
<ul>
<li><a href="#initializing-other-state"
id="toc-initializing-other-state">Initializing Other State</a></li>
<li><a href="#stubbing-external-calls"
id="toc-stubbing-external-calls">Stubbing External Calls</a></li>
<li><a href="#wrapping-up" id="toc-wrapping-up">Wrapping Up</a></li>
<li><a href="#optional-exercise-patch-the-placement-algorithm"
id="toc-optional-exercise-patch-the-placement-algorithm">Optional
Exercise: Patch the Placement Algorithm</a></li>
</ul></li>
<li><a href="#debugging-p-code-semantics"
id="toc-debugging-p-code-semantics">Debugging P-code Semantics</a></li>
</ul></li>
</ul>
</nav>
<section id="emulation" class="level1">
<h1>Emulation</h1>
<p>Emulation is a bit of a loaded term, unfortunately. Most of the
confusion deals with the scope of the emulated target. Do you just need
to step through a few instructions, or a whole function? Do you need to
include external modules? Do you need to simulate system calls? Do you
need to simulate connected devices? Most of Ghidra’s GUI-accessible
emulation features focus on the smaller scope, though it does provide
programming interfaces for advanced users to extend that scope. Those
more advanced features are covered in <a
href="B4-Modeling.html">Modeling</a>.</p>
<p>This module assumes you have completed the Beginner portion of this
course.</p>
<section id="p-code-emulation-and-caveats" class="level2">
<h2>P-code Emulation and Caveats</h2>
<p>Ghidra’s emulator uses the same p-code as is used by the decompiler.
P-code describes the semantics of each instruction by constructing a
sequence of p-code operations. The p-code specifications for most of
Ghidra’s languages were designed with decompilation, not necessarily
emulation, in mind. While in most cases, p-code for decompilation
suffices for emulation, there are cases where design decisions were
made, e.g., to keep decompiler output simple, that makes them less
suitable for emulation. This may manifest, e.g., in an excess of
user-defined p-code ops, or <em>userops</em>. The <a
href="B4-Modeling.html">Modeling</a> module discusses ways to implement
or stub those userops in the emulator. Some processor modules provide
those stubs “out of the box.” If the emulator ever halts with an
“unimplemented userop” message, then you have run into this problem.</p>
</section>
<section id="use-cases" class="level2">
<h2>Use Cases</h2>
<p>As already hinted at the start of this module, there are several use
cases for emulation, and Ghidra tries to meet these cases by integrating
emulation into the Debugger UI. Some of the use cases accessible from
the UI are:</p>
<ul>
<li>Extrapolation and interpolation of a live target.</li>
<li>Emulation of a program image.</li>
<li>P-code semantics debugging.</li>
</ul>
<p>We will explore each case with a tutorial and exercise.</p>
</section>
<section id="extrapolation-and-interpolation" class="level2">
<h2>Extrapolation and Interpolation</h2>
<p>This is perhaps the easiest use case, assuming you already have
started a live session. <em>Extrapolation</em> is predicting execution
of the target into the future, without allowing the actual target to
execute. Instead, we will allow an emulator to step forward, while
reading its initial state from the live target. This allows you, e.g.,
to experiment with various patches, or to force execution down a certain
path. If you devise a patch, you can then apply it the live target and
allow it to execute for real. <em>Interpolation</em> is similar, but
from a snapshot that is in the past. It can help answer the question,
“How did I get here?” It is more limited, because missing state for
snapshots in the past cannot be recovered.</p>
<p>In this tutorial, we will examine the command-line argument parser in
<code>termmines</code>.</p>
<ol type="1">
<li>Launch <code>termmines</code> using GDB in the Ghidra Debugger.</li>
<li>If you have not already, do a bit of static analysis to identify the
argument parsing function. It should be the first function called by
<code>main</code>.</li>
<li>Use a breakpoint to interrupt the live target when it enters this
function.</li>
<li>Change the <strong>Control mode</strong> drop-down to
<strong>Control Emulator</strong>.</li>
<li>Click <img src="images/stepinto.png" alt="step into button" />
<strong>Step Into</strong> to step the emulator forward.</li>
<li>Click <img src="images/skipover.png" alt="skip over button" />
<strong>Skip Over</strong> and <img src="images/stepback.png"
alt="step back button" /> <strong>Step Back</strong> to experiment with
different execution paths.</li>
</ol>
<p>About those two new actions:</p>
<ul>
<li><img src="images/skipover.png" alt="skip over button" />
<strong>Skip Over</strong>: Step the current thread by skipping one
instruction.</li>
<li><img src="images/stepback.png" alt="step back button" />
<strong>Step Back</strong>: Step the current thread backward one
instruction, or undo an emulated skip or patch.</li>
</ul>
<p><strong>Quick Exercise</strong>: Try to get the program counter onto
the call to <code>exit(-1)</code> using only those three step
buttons.</p>
<p>You should see things behave more or less the same as they would if
it were the live target. The main exception is the Terminal window. It
always displays the state of the live target, as it is unaware of the
emulator. You can make changes to the emulator’s machine state, set
breakpoints, etc., just as you would in <strong>Control Target</strong>
mode. <strong>NOTE</strong>: You may see Ghidra interact with the
target, despite being in <strong>Control Emulator</strong> mode, because
Ghidra lazily initializes the emulator’s state. If the emulated target
reads a variable that Ghidra has not yet captured into the current
snapshot, Ghidra will read that variable from the live target, capture
it, and provide its value to the emulator.</p>
<section id="stepping-schedules" class="level3">
<h3>Stepping Schedules</h3>
<p>If you had not noticed before, the trace tabs include the current
snapshot number. If you have stepped in the emulator, it will also
contain the sequence of steps emulated. Recall the <em>time</em> element
of the Debugger’s <em>coordinates</em>. (See the <a
href="A5-Navigation.html">Navigation</a> module if you need a
refresher.) The time element, called the <em>schedule</em>, consists of
both the current snapshot and the sequence of steps to emulate. The tab
displays that schedule. If you have done any patching of the emulator’s
state, you may notice some more complicated “steps” in the schedule. The
syntax is:</p>
<ul>
<li><em>Schedule</em> → <em>Snapshot</em> [ <code>:</code> [
<em>Step</em> ( <code>;</code> <em>Step</em> ) * ] [ <code>.</code>
<em>Step</em> ( <code>;</code> <em>Step</em> ) * ] ]</li>
<li><em>Step</em> → [ <code>t</code> <em>Id</em> <code>-</code> ] (
<em>Tick</em> | <em>Skip</em> | <em>Patch</em> )</li>
<li><em>Tick</em> → <em>Count</em></li>
<li><em>Skip</em> → <code>s</code> <em>Count</em></li>
<li><em>Patch</em> → <code>{</code> <em>SleighStmt</em>
<code>}</code></li>
</ul>
<p>In essence, the schedule is the starting snapshot, followed by zero
or more machine-instruction steps followed by zero or more
p-code-operation steps. Each step is optionally preceded by a thread id.
If omitted, the thread id is the same as the previous step. If the first
step has no thread id, it applies to the snapshot’s event thread. A
plain number indicates the number of instructions or operations to
execute. An <code>s</code> prefix indicates skip instead of execute.
Curly braces specify a patch using a single Sleigh statement. Here are
some examples:</p>
<ul>
<li><code>0</code> — The first snapshot in the trace.</li>
<li><code>3</code> — Snapshot number 3.</li>
<li><code>3:10</code> — Emulate 10 machine instructions on the event
thread, starting at snapshot 3.</li>
<li><code>3:t1-10</code> — Same as above, but on the second thread
rather than the event thread.</li>
<li><code>3:10;t1-10</code> — Start at snapshot 3. Step the event thread
10 instructions. Step the second thread 10 instructions.</li>
<li><code>3:10.4</code> — Start at snapshot 3. Step the event thread 10
instructions then 4 p-code ops.</li>
<li><code>3:{RAX=0x1234};10</code> — Start at snapshot 3. Override RAX
with 0x1234, then step 10 instructions.</li>
</ul>
<p>The explication of schedules allows Ghidra to cache emulated machine
states and manage its emulators internally. You can have Ghidra recall
or generate the machine state for any schedule by pressing
<strong><code>CTRL</code>-<code>G</code></strong> or using
<strong>Debugger → Go To Time</strong> in the menus.</p>
<p>Assuming you got the program counter onto <code>exit(-1)</code>
earlier:</p>
<ol type="1">
<li>Write down the current schedule.</li>
<li>Change back to <strong>Control Target</strong> mode. Ghidra will
navigate back to the current snapshot, so PC will match the live
target.</li>
<li>Change back (again) to <strong>Control Emulator</strong> mode.</li>
<li>Press <strong><code>CTRL</code>-<code>G</code></strong> and type or
paste the schedule in, and click <strong>OK</strong>. The program
counter should be restored to <code>exit(-1)</code>.</li>
</ol>
<p><strong>NOTE</strong>: The thread IDs used in schedules are internal
to the current trace database. Most likely, they <em>do not</em>
correspond to the thread IDs assigned by the back-end debugger.</p>
</section>
<section id="exercise-demonstrate-the-cell-numbers" class="level3">
<h3>Exercise: Demonstrate the Cell Numbers</h3>
<p>The board setup routine in <code>termmines</code> first places mines
randomly and then, for each empty cell, counts the number of neighboring
cells with mines. In this exercise, you will use extrapolation to
experiment and devise a patch to demonstrate all possible counts of
neighboring mines:</p>
<ol type="1">
<li>Launch <code>termmines</code> using GDB with <strong>Inferior
TTY</strong> enabled.</li>
<li>Use a breakpoint to trap it at the point where it has placed mines,
but before it has counted the neighboring cells with mines. (Use
<strong><code>SHIFT</code>-<code>R</code></strong> in
<code>termmines</code> to reset the game.)</li>
<li>Use the emulator to extrapolate forward and begin understanding how
the algorithm works.</li>
<li>Move the mines by patching the board to demonstrate every number of
neighboring mines. That is, when the board is revealed at the end of the
game, all the numbers 1 through 8 should appear somewhere.</li>
<li>Use extrapolation to debug and test your patch.</li>
<li>Once you have devised your patch, apply it to the live target.
(Copy-Paste is probably the easiest way to transfer the state from
emulator to target.)</li>
</ol>
</section>
</section>
<section id="emulating-a-program-image" class="level2">
<h2>Emulating a Program Image</h2>
<p>This use case allows you to load “any” Ghidra program database into
the emulator, without a back-end debugger, host environment, or other
dependencies. The result and efficacy of this method depends greatly on
what is captured in the program database. When Ghidra imports an ELF
file, it simulates the OS’s loader, but only to a degree: It places each
section at its load memory address, it applies relocation fixups, etc.
The resulting program database is suitable for emulating that image, but
in relative isolation. It is probably not possible to load a library
module into that same database nor into the same emulator and expect
proper linkage. Ghidra’s loaders often “fix up” references to external
symbols by allocating a special <code>EXTERNAL</code> block, and placing
the external symbols there. There is (currently) no means to re-fix up.
If, however, you import a firmware image for an embedded device, or a
memory dump of a process, then the image may already have all the code
and linkage necessary.</p>
<p>It is too tedious to categorize every possible situation and failure
mode here. When you encounter an error, you should diagnose it with
particular attention to the contents of your program image, and how it
expects to interact with its environment: the host system, connected
hardware, etc. The UI has some facilities to stub out dependencies, but
if you find yourself creating and applying an extensive suite of stubs,
you may want to consider <a href="B4-Modeling.html">Modeling</a>. This
allows you to code your stubs into a library, facilitating re-use and
repeatability.</p>
<p>Emulation need not start at the image’s designated entry point. In
this tutorial, we will examine the command-line argument parsing
routine.</p>
<ol type="1">
<li>Ensure you have no active targets in the Debugger, but have
<code>termmines</code> open in the Static listing.</li>
<li>Go to the entry of the command-line argument parsing function.</li>
<li>Right-click its first instruction and select <strong>Emulate Program
in New Trace</strong>.</li>
</ol>
<p>This will map the program into a new trace. Technically, it is not
actually loaded into an emulator, yet, because Ghidra allocates and
caches emulators as needed. Instead, what you have is a single-snapshot
trace without a live target. The initial state is snapshot 0, and
emulation is started by navigating to a schedule, just like in
extrapolation.</p>
<figure>
<img src="images/Emulation_InitialListing.png"
alt="Listing upon starting pure emulation" />
<figcaption aria-hidden="true">Listing upon starting pure
emulation</figcaption>
</figure>
<p>Because we can easily step back and forth as well as navigate to
arbitrary points in time, emulation should feel relatively free of risk;
however, the point about stubbing dependencies will become apparent. If
you feel the need to start over, there are two methods: First, you can
end the emulation session and restart it. To end the session, close the
“Emulate termmines” tab in the Dynamic Listing window. You can then
restart by right-clicking the first instruction as before. Second, you
can use <strong><code>CTRL</code>-<code>G</code></strong> to go to
snapshot 0. This method is not as clean as the first, because the trace
will retain its scratch snapshots.</p>
<p>Press <img src="images/resume.png" alt="resume button" />
<strong>Resume</strong> to let the emulator run until it crashes. It
should crash pretty quickly and without much ceremony:</p>
<figure>
<img src="images/Emulation_ListingAfterResume.png"
alt="Listing after crashing" />
<figcaption aria-hidden="true">Listing after crashing</figcaption>
</figure>
<p>In this case, the clearest indication that something has gone wrong
is in the top-right of the Dynamic Listing. Recall that the location
label is displayed in red when the program counter points outside of
mapped memory. Presumably, the crash was caused by the instruction to be
executed next. To get details about the error, press <img
src="images/stepinto.png" alt="step into button" /> <strong>Step
Into</strong>. This should display an error dialog with a full trace of
the crash. In this case, it should be an instruction decode error. When
the emulator reads uninitialized memory, it will get stale 0s; however,
when the emulator tries to <em>execute</em> uninitialized memory, it
will crash. Most likely, the target called an external function, causing
the program counter to land in the fake <code>EXTERNAL</code> block.</p>
<p>To diagnose the crash, press <img src="images/stepback.png"
alt="step back button" /> <strong>Step Back</strong>. After a couple
steps back, you should be able to confirm our hypothesis: we got here
through a call to the external function <code>printf</code>. You can
continue stepping back until you find the decision point that took us
down this path. You should notice it was because <code>param_1</code>
was 0. The decompiler can help you recognize that at a glance, but you
will still want to use the disassembly to get at precisely the deciding
instruction. The <code>JZ</code> (or other conditional jump) is too
late; you need to step back to the <code>TEST EDI,EDI</code> (or
similar) instruction. (This may, ironically, be the first instruction of
the function.) In the System V AMD64 ABI (Linux x86-64 calling
conventions) <code>RDI</code> is used to pass the first parameter. You
can hover your mouse over <code>param_1</code> in the Decompiler, and it
will tell you the location is <code>EDI:4</code>, and that its current
value is a stale 0.</p>
<section id="initializing-other-state" class="level3">
<h3>Initializing Other State</h3>
<p>We had just started executing the target function arbitrarily. Ghidra
takes care of a minimal bit of initialization of the trace to start
emulation. Namely, it maps the image to its preferred base. It allocates
space for the main thread’s stack and initializes the stack pointer.
Finally, it initializes the program counter.</p>
<p>It is still up to you to initialize any other state, especially the
function’s parameters. Clearly, we will need to initialize
<code>param_1</code>. We may need to do a little static analysis around
the call to this function to understand what those parameters are, but
you could probably make an educated guess: <code>param_1</code> is
<code>argc</code> and <code>param_2</code> is <code>argv</code>. We
might as well initialize both. Luckily, we have plenty of memory, and
given the small scope of emulation, we can probably place the strings
for <code>argv</code> wherever we would like.</p>
<p>You may prefer to apply patches to the trace database or to the
emulator. The advantage to patching in the emulator is that once you
have completed your experiments, you can readily see all of the steps
that got you to the current machine state, including all patches. The
disadvantage is that if you have extensive patches, they will pollute
the stepping schedule, and things can get unwieldy.</p>
<p>Alternatively, you can perform the patches in the trace. When you
launched the emulated target, all Ghidra really did was initialize a
trace database. The advantage to patching the trace is that once you
have completed your experiments, you will have your initial state
captured in a trace snapshot. The disadvantage is that you will need to
remember to invalidate the emulator cache any time you change the
initial state. For this tutorial, we will perform the patches in the
emulator.</p>
<p><strong>NOTE</strong>: If you wish to try patching the trace, then
change to <strong>Control Trace</strong> mode and use the
<strong>Navigate backward one snapshot</strong> control action that
appears, so that you are patching the initial state, and not a scratch
snapshot. Scratch snapshots are ephemeral snapshots in the trace used to
display emulated state. Changes to these snapshots will affect the
display, but will not affect subsequent emulation. If your current
schedule includes any steps, then <strong>Control Trace</strong> is
patching a scratch snapshot.</p>
<p>Now, we will manually “allocate” memory for <code>argv</code>.
Luckily, Ghidra allocated 16K of stack space for us! The target function
should not need a full 16K, so we will allocate the lowest addresses of
the stack region for our command-line arguments. If you prefer, you may
use the <strong>Add Region</strong> action in the Regions window to
manually fabricate a heap region, instead. In the Regions window, filter
for “stack” and take note of the start address, e.g.,
<code>00001000</code>. We will use the Watches window to perform our
patching, though we will also use the Dynamic Listing to double check.
Add the following watches:</p>
<ul>
<li><code>RSP</code> — to confirm the stack pointer is far from
<code>argv</code>.</li>
<li><code>RDI</code> — the location of <code>param_1</code>, i.e.,
<code>argc</code>.</li>
<li><code>RSI</code> — the location of <code>param_2</code>, i.e.,
<code>argv</code>.</li>
</ul>
<p>To start, we will just try to return successfully from the parser.
From the behavior we have observed, it requires at least
<code>argv[0]</code> to be present. Conventionally, this is the name of
the binary as it was invoked from the shell, i.e.,
<code>termmines</code>. There are few reasons a UNIX program might want
to examine this “argument.” First, if the binary actually implements
many commands, like <code>busybox</code> does, then that binary needs to
know the actual command. Second, if the binary needs to print usage
information, it may like to echo back the actual invocation. It is
possible we may only need to initialize <code>argc</code>, since the
parser may not actually <em>use</em> the value of
<code>argv[0]</code>.</p>
<p>Use the Watches window to set <code>RDI</code> to 1, then click <img
src="images/resume.png" alt="resume button" /> <strong>Resume</strong>.
Like before, the emulator will crash, but this time you should see “pc =
00000000” in red. This probably indicates success. In the Threads
window, you should see a schedule similar to
<code>0:t0-{RDI=0x1);t0-16</code>. This tells us we first patched RDI,
then emulated 16 machine instructions before crashing. When the parser
function returned, it probably read a stale 0 as the return address, so
we would expect a decode error at <code>00000000</code>. Step backward
once to confirm this hypothesis.</p>
</section>
<section id="stubbing-external-calls" class="level3">
<h3>Stubbing External Calls</h3>
<p>For this tutorial, we will set the skill level to Advanced by
patching in actual command-line arguments. This continues our lesson in
state initialization, but we may also need to stub some external calls,
e.g., to <code>strnlen</code> and <code>strcmp</code>. We will need to
pass in <code>termmines -s Advanced</code>, which is three arguments.
Use <strong><code>CTRL</code>-<code>G</code></strong> to go back to
snapshot 0, and add the following watches:</p>
<ul>
<li><code>*:8 (RSI + 0)</code> — the address of the first argument,
i.e., <code>argv[0]</code>.</li>
<li><code>*:30 (*:8 (RSI + 0))</code> with type
<code>TerminatedCString</code> — at most 30 characters of the first
argument.</li>
<li><code>*:8 (RSI + 8)</code> — <code>argv[1]</code></li>
<li><code>*:30 (*:8 (RSI + 8))</code> with type
<code>TerminatedCString</code> — contents of <code>argv[1]</code></li>
<li><code>*:8 (RSI + 16)</code> — <code>argv[2]</code></li>
<li><code>*:30 (*:8 (RSI + 16))</code> with type
<code>TerminatedCString</code> — contents of <code>argv[2]</code></li>
</ul>
<figure>
<img src="images/Emulation_WatchesForCmdline.png"
alt="Watches for patching command-line arguments" />
<figcaption aria-hidden="true">Watches for patching command-line
arguments</figcaption>
</figure>
<p>This will generate an extensive list of patch steps, so you may
prefer to patch the trace in this case. Set <code>RDI</code> to 3.
Notice that <code>argv[0]</code> is supposedly allocated at
<code>00000000</code> according to the Address column for the watch on
<code>*:8 (RSI + 0)</code>. That was determined by the value of
<code>RSI</code>, which is essentially telling us we need to allocate
<code>argv</code>, an array of pointers. We can confirm <code>RSP</code>
is at the upper end of the stack region, so we allocate
<code>argv</code> at <code>00001000</code>. To do that, set the value of
<code>RSI</code> to <code>0x1000</code>. You should see the Address
column update for some other watches. You can double-click any of those
addresses to go there in the Dynamic Listing.</p>
<p><strong>NOTE</strong>: You <em>do not have</em> to allocate things in
a listed region, but if you want to see those things in the Dynamic
Listing, it is easiest if you allocate them in a listed region.</p>
<p>Now, we need to allocate space for each argument’s string. To ensure
we do not collide with the space we have already allocated for
<code>argv</code>, we should place a data unit in the Dynamic listing.
Double-click the Address <code>00001000</code> in the Watches window to
go to that address in the Dynamic Listing. Press
<strong><code>P</code></strong> then <strong><code>[</code></strong>
(left square bracket) to place a 3-pointer array at that address. We can
now see the next available byte is at <code>00001018</code>.
<strong>NOTE</strong>: You might set the Dynamic Listing to <strong>Do
Not Track</strong>, otherwise it may seek back to the PC every time you
patch.</p>
<p>Now that we know where to put <code>argv[0]</code>, we need to patch
it to <code>0x0001018</code>. This should be the watch on
<code>*:8 (RSI + 0)</code>. When you modify the Value column, you can
type either bytes (in little-endian order for x86) or the integer value
<code>0x1018</code>. That should cause the watch on
<code>*:30 (*:8 (RSI + 0))</code> to get the address
<code>00001018</code>. Using the Repr column, set that watch’s value to
<code>"termmines"</code>. (The quotes are required.) Place a string in
the Dynamic Listing using the <strong><code>'</code></strong>
(apostrophe) key. This shows us the next available address is
<code>00001022</code>, so repeat the process to allocate
<code>argv[1]</code> and set it to <code>"-s"</code>. Then finally,
allocate <code>argv[2]</code> and set it to <code>"Advanced"</code>.
When you have finished, the Watches pane should look something like
this:</p>
<figure>
<img src="images/Emulation_WatchesForCmdlineSet.png"
alt="Watches for patching command-line arguments after setting" />
<figcaption aria-hidden="true">Watches for patching command-line
arguments after setting</figcaption>
</figure>
<p>The Dynamic Listing should look something like this:</p>
<figure>
<img src="images/Emulation_ListingForCmdlineSet.png"
alt="Listing after setting command-line arguments" />
<figcaption aria-hidden="true">Listing after setting command-line
arguments</figcaption>
</figure>
<p><strong>NOTE</strong>: The placement of data units is not necessary
for the emulator to operate; it only cares about the bytes. However, it
is a useful aide in devising, understanding, and diagnosing machine
state.</p>
<p>Now, click <img src="images/resume.png" alt="resume button" />
<strong>Resume</strong>, and see where the emulator crashes next.
Depending on your compilation of <code>termmines</code>, it may crash
after returning, or it may crash trying to call <code>strnlen</code> or
<code>strcmp</code>. If the program counter is <code>00000000</code>,
then it returned successfully. This is unfortunate, because you no
longer have motivation to stub external calls.</p>
<p>If the program counter is not <code>00000000</code>, then step
backward until you get to the <code>CALL</code>. There are at least
three techniques for overcoming this.</p>
<ol type="1">
<li>You can skip the <code>CALL</code> and patch <code>RAX</code>
accordingly.</li>
<li>You can override the <code>CALL</code> instruction using a Sleigh
breakpoint.</li>
<li>You can override the call target using a Sleigh breakpoint.</li>
</ol>
<section id="skip-technique" class="level4">
<h4>Skip Technique</h4>
<p>The skip technique is simplest, but will need to be performed
<em>every time</em> that call is encountered. Press <img
src="images/skipover.png" alt="skip over button" /> <strong>Skip
Over</strong>, then use the Registers or Watches pane to patch
<code>RAX</code>. Then press <img src="images/resume.png"
alt="resume button" /> <strong>Resume</strong>.</p>
</section>
<section id="call-override-technique" class="level4">
<h4><code>CALL</code> Override Technique</h4>
<p>Overriding the <code>CALL</code> is also fairly simple. While this
will handle every encounter, it will not handle other calls to the same
external function.</p>
<ol type="1">
<li>Press <strong><code>K</code></strong> in the listing to place a
breakpoint on the <code>CALL</code> instruction.</li>
<li>Now, in the Breakpoints panel, right-click the new breakpoint and
select <strong>Set Injection (Emulator)</strong>.</li>
<li>This is the fun part: you must now implement the function in Sleigh,
or at least stub it well enough for this particular call.</li>
</ol>
<p>Supposing this is a call to <code>strnlen</code>, you could implement
it as:</p>
<div class="sourceCode" id="cb1"><pre
class="sourceCode numberSource sleigh numberLines"><code class="sourceCode sleighsyntax"><span id="cb1-1"><a href="#cb1-1"></a>RAX = <span class="dv">0</span>;</span>
<span id="cb1-2"><a href="#cb1-2"></a>&lt;<span class="at">loop</span>&gt;</span>
<span id="cb1-3"><a href="#cb1-3"></a><span class="cf">if</span> (*:<span class="dv">1</span> (RDI+RAX) == <span class="dv">0</span> || RAX &gt;= RSI) <span class="cf">goto</span> &lt;<span class="at">exit</span>&gt;;</span>
<span id="cb1-4"><a href="#cb1-4"></a>RAX = RAX + <span class="dv">1</span>;</span>
<span id="cb1-5"><a href="#cb1-5"></a><span class="cf">goto</span> &lt;<span class="at">loop</span>&gt;;</span>
<span id="cb1-6"><a href="#cb1-6"></a>&lt;<span class="at">exit</span>&gt;</span>
<span id="cb1-7"><a href="#cb1-7"></a>emu_skip_decoded();</span></code></pre></div>
<p>While Sleigh has fairly nice C-like expressions, it unfortunately
does not have C-like control structures. We are essentially writing a
for loop. The System V AMD64 ABI specifies RAX is for the return value,
so we can just use it directly as the counter. RDI points to the string
to measure, and RSI gives the maximum length. We initialize RAX to 0,
and then check if the current character is NULL, or the count has
exceeded the maximum length. If so, we are done; if not, we increment
RAX and repeat. Finally, because we are <em>replacing</em> the semantics
of the <code>CALL</code> instruction, we tell the emulator to skip the
current instruction.</p>
<p>For the complete specification of Sleigh, see the Semantic Section in
the <a
href="../../../Ghidra/Features/Decompiler/src/main/doc/sleigh.xml">Sleigh
documentation</a>. The emulator adds a few userops:</p>
<ul>
<li><code>emu_skip_decoded()</code>: Skip the current instruction.</li>
<li><code>emu_exec_decoded()</code>: Execute the current
instruction.</li>
<li><code>emu_swi()</code>: Interrupt, as in a breakpoint.</li>
</ul>
<p>Some control flow is required in the Sleigh injection, otherwise, the
emulator may never advance past the current instruction. An explicit
call to <code>emu_exec_decoded()</code> allows you to insert logic
before and/or after the original instruction; however, if the original
instruction branches, then the logic you placed <em>after</em> will not
be reached. An explicit call to <code>emu_skip_decoded()</code> allows
you to omit the original instruction altogether. It immediately falls
through to the next instruction. The <code>emu_swi()</code> userop
allows you to maintain breakpoint behavior, perhaps to debug your
injection.</p>
<p>After you have written your Sleigh code:</p>
<ol type="1">
<li>Click <strong>OK</strong> on the Set Injection dialog.</li>
<li>In the menus, select <strong>Debugger → Configure Emulator →
Invalidate Emulator Cache</strong>.</li>
<li>Click <img src="images/resume.png" alt="resume button" />
<strong>Resume</strong>.</li>
</ol>
<p>Stubbing any remaining external calls is left as an exercise. You are
successful when the emulator crashes with “pc = 00000000”.</p>
<p>Clear or disable your breakpoint and invalidate the emulator cache
again before proceeding to the next technique.</p>
</section>
<section id="target-override-technique" class="level4">
<h4>Target Override Technique</h4>
<p>The target override technique is most thorough, but also the most
involved. It will handle all calls to the external function, e.g.,
<code>strnlen</code>, no matter the call site. If the call goes through
a program linkage table (PLT), then you are in luck, because the call
target will be visible in the Dynamic Listing. The PLT entry usually
contains a single <code>JMP</code> instruction to the actual
<code>strnlen</code>. For real target processes, the <code>JMP</code>
instruction will transfer control to a lazy linker the first time
<code>strnlen</code> is called from <code>termmines</code>. The linker
then finds <code>strnlen</code> and patches the table. In contrast, the
Ghidra loader immediately patches the table to point to a fake
<code>&lt;EXTERNAL&gt;::strnlen</code> symbol. The <code>EXTERNAL</code>
block is not visible in the Dynamic Listing, so we will override the
<code>JMP</code> in the PLT.</p>
<p>The Sleigh code is nearly identical, but we must code an x86
<code>RET</code> into it. Because we allow the <code>CALL</code> to
execute normally, we must restore the stack. Furthermore, we must return
control back to the caller, just like a real x86 subroutine would. We
also no longer need <code>emu_skip_decoded()</code>, because the
<code>RET</code> will provide the necessary control transfer.</p>
<div class="sourceCode" id="cb2"><pre
class="sourceCode numberSource sleigh numberLines"><code class="sourceCode sleighsyntax"><span id="cb2-1"><a href="#cb2-1"></a>RAX = <span class="dv">0</span>;</span>
<span id="cb2-2"><a href="#cb2-2"></a>&lt;<span class="at">loop</span>&gt;</span>
<span id="cb2-3"><a href="#cb2-3"></a><span class="cf">if</span> (*:<span class="dv">1</span> (RDI+RAX) == <span class="dv">0</span> || RAX &gt;= RSI) <span class="cf">goto</span> &lt;<span class="at">exit</span>&gt;;</span>
<span id="cb2-4"><a href="#cb2-4"></a>RAX = RAX + <span class="dv">1</span>;</span>
<span id="cb2-5"><a href="#cb2-5"></a><span class="cf">goto</span> &lt;<span class="at">loop</span>&gt;;</span>
<span id="cb2-6"><a href="#cb2-6"></a>&lt;<span class="at">exit</span>&gt;</span>
<span id="cb2-7"><a href="#cb2-7"></a>RIP = *:<span class="dv">8</span> RSP;</span>
<span id="cb2-8"><a href="#cb2-8"></a>RSP = RSP + <span class="dv">8</span>;</span>
<span id="cb2-9"><a href="#cb2-9"></a><span class="cf">return</span> [RIP];</span></code></pre></div>
<p>Notice that we cannot just write <code>RET</code>, but instead must
write the Sleigh code to mimic a <code>RET</code>. As with the
<code>CALL</code> override technique, you must now invalidate the
emulator cache and resume. Stubbing any remaining external functions is
left as an exercise. You are successful when the emulator crashes with
“pc = 00000000”.</p>
</section>
</section>
<section id="wrapping-up" class="level3">
<h3>Wrapping Up</h3>
<p>As you can see, depending on the scope of emulation, and the
particulars of the target function, emulating a program image can be
quite involved. Whatever technique you choose, once you have
successfully returned from the command-line argument parser, you should
check for the expected effects.</p>
<p>In the Static Listing, navigate to the variable that stores the
board’s dimensions. (Finding that variable is a task in the Beginner
portion, but it can be found pretty easily with some manual static
analysis.) In the Dynamic Listing, you should notice that the values
have changed to reflect the Advanced skill level.</p>
</section>
<section id="optional-exercise-patch-the-placement-algorithm"
class="level3">
<h3>Optional Exercise: Patch the Placement Algorithm</h3>
<p>In this exercise, you will use emulation to devise an assembly patch
to <code>termmines</code> to change the mine placement algorithm.
Instead of random placement, please have them placed left to right, top
to bottom. We recommend you devise your patch using the Assembler
(<strong>Patch Instruction</strong> action) in the Static Listing, then
test and debug your patch using the Emulator. Perhaps patch the Dynamic
Listing to try quick tweaks before committing them to the Static
Listing. Once you have it, export the patched binary and run it outside
of Ghidra.</p>
</section>
</section>
<section id="debugging-p-code-semantics" class="level2">
<h2>Debugging P-code Semantics</h2>
<p>The last use case for emulation we will cover in this course is
debugging p-code semantics. This use case is a bit niche, so we will not
cover it too deeply. It is useful for debugging processor modules. It is
also useful in system modeling, since a lot of that is accomplished
using Sleigh p-code. Perhaps the most useful case related to this module
is to debug Sleigh injections.</p>
<p>Ghidra has a dedicated panel for stepping the emulator one p-code
operation at a time. This panel is not included in the default Debugger
tool, so it must be configured:</p>
<ol type="1">
<li>If you have not already, open the Debugger tool.</li>
<li>In the menus, select <strong>File → Configure</strong>.</li>
<li>Click the <strong>Configure All Plugins</strong> button in the top
right of the dialog.</li>
<li>Activate the <code>DebuggerPcodeStepperPlugin</code></li>
<li>Click <strong>OK</strong></li>
<li>Click <strong>Close</strong></li>
</ol>
<p>The stepper should appear stacked over the Threads panel in the
bottom right. Yours will probably still be empty, but here is what it
looks like populated:</p>
<figure>
<img src="images/Emulation_PcodeStepper.png" alt="P-code stepper" />
<figcaption aria-hidden="true">P-code stepper</figcaption>
</figure>
<p>To populate it, you will need a session, either emulated or connected
to a back-end debugger. Use the buttons in the local toolbar to step
p-code operations. The first p-code op of any instruction is to decode
the instruction. Once decoded, the p-code listing (left panel) will
populate with the ops of the decoded instruction. If the current
instruction is overridden by a Sleigh breakpoint, the listing will
populate with the injected ops instead. You can then step forward and
backward within those. As you step, the other windows that display
machine state will update.</p>
<p>In addition to registers and memory, p-code has <em>unique</em>
variables. These are temporary variables used only within an
instruction’s implementation. They are displayed in the right panel. The
table of variables works similarly to the Registers pane. The columns
are:</p>
<ul>
<li>The <strong>Unique</strong> column gives the variable’s name and
size in bytes.</li>
<li>The <strong>Bytes</strong> column gives the variable’s value in
bytes.</li>
<li>The <strong>Value</strong> column gives the variable’s value as an
integer, an interpretation of the bytes in the machine’s byte
order.</li>
<li>The <strong>Type</strong> column allows you to assign a type. This
is ephemeral.</li>
<li>The <strong>Repr</strong> column gives the variable’s value
according to the assigned type.</li>
</ul>
<p>As you step, you may notice the schedule changes. It is displayed in
the stepper’s subtitle as well as the Threads panel’s subtitle. P-code
stepping is denoted by the portion of the schedule following the dot.
<strong>NOTE</strong>: You cannot mix instruction steps with p-code op
steps. The instruction steps always precede the p-code ops. If you click
<strong>Step Into</strong> from the global toolbar in the middle of an
instruction, the trailing p-code op steps will be removed and replaced
with a single instruction step. In most cases, this intuitively
“finishes” the partial instruction.</p>
</section>
</section>
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